1. Field of the Invention.
The present invention relates generally to a magnetic position encoder system for sensing the angular position of a moving member, and relates more particularly to a magnetic rotary encoder system having a magnetic sensor and an encoder wheel wherein the magnetic sensor includes an array of magnetoresistive (MR) elements arranged such as to produce one or more channels of incremental signals which are related directly to the angular position of recorded information on one or more tracks on the encoder wheel. Each track of the information is recorded as a magnetic pole array on the encoder wheel, such that as the wheel attached to the moving member rotates, the orientation of the resulting magnetic field at each MR element changes in such a way as to modify the resistance of the element.
2. Background Art
It is well known in the art that when an MR element made of material such as permalloy is placed in a magnetic field, the electrical resistance of the MR element changes depending upon the strength of the magnetic field. Thus, when such an MR element is used to sense magnetic information recorded on a magnetic medium, the recorded magnetic information can be reliably sensed even when the relative velocity between the magnetic medium and the MR element is zero. A transducer apparatus comprising an array of MR elements and using the principle of the detection of the resistance of these MR elements in the presence of a magnetic field to determine the relative displacement of the medium carrying the magnetic information is disclosed, for example, in U.S. Pat. Nos. 4,039,936, 4,403,187, 4,418,372, 4,628,259, 4,639,807, 4,731,580, and 4,853,631.
It is also known in the art, and specifically from U.S. Pat. No. 4,039,936 issued Aug. 2, 1977 to A. L. Jones et al., that a change in a voltage signal can be derived from the changes in resistance in the MR elements if the MR elements are grouped in sections and if each section is symmetrically disposed at a pitch which is related to the pitch of the magnetic pole array of the corresponding track on the encoder wheel. Each of these sections may be a group of two or more MR elements, including serpentine shapes which are used to increase the total length and resistance of the transducer. A significant limitation of the magnetic field system described in U.S. Pat. No. 4,039,936 is that relative motion is established between a single permanent magnet pole to which the MR element array is attached and a toothed rack made of a soft ferromagnetic material; each section of the MR elements is symmetrically disposed at the half-pitch of the teeth in the rack and parallel with the tooth edges. The resolution of angular position using this scheme is limited by the need of each MR element to clearly discriminate between the gaps from the permanent magnet to a tooth and from the permanent magnet to a slot in the rack; the pitch of the teeth in the rack needs to be large compared to the separation between the teeth and the MR element array for this to be achieved. This limitation is aggravated by the almost uni-directional orientation of the magnetic field which is perpendicular to the plane of the MR elements; the maximum field strength is experienced in any one of the MR elements when it is in the center of a gap between the permanent magnet and a tooth, whereas the minimum field strength is experienced when an MR element is in the center of a gap between the permanent magnet and a slot in the rack.
It is well known in the art that when an MR element made of material such as permalloy is placed with its plane perpendicular to a magnetic field, the greatest change in electrical resistance of the MR element is achieved when the minimum field experienced by the MR element is of insufficient strength to penetrate the MR element while the maximum field experienced by the MR element is of sufficient strength to saturate all of the MR element material. The greatest possible change in electrical resistance of an MR element is only realized when the magnetic field fully saturates the MR material in one position and when it is completely excluded from the MR material in another position. Using an almost uni-directional field from a permanent magnet into a toothed rack such as that described in U.S. Pat. No. 4,039,936, this requirement imposes a serious limitation on the minimum pitch that can be employed for the teeth in that rack, and hence a limitation on the resolution of angular position that can be achieved by a magnetic position encoder using this principle.
Accordingly, it is a primary object of the present invention to provide a magnetic sensor comprising an array of MR elements and an encoder wheel which carries one or more magnetic pole arrays, such that as the wheel rotates relative to the sensor, the orientation of the magnetic field experienced by an MR element causes a sufficient change in the resistance of that MR element to provide an electrical signal from which the incremental angular rotation of the wheel can be deduced.
It is a further object of the present invention to provide an MR element and an encoder wheel which carries a magnetic pole array, such that as the wheel rotates relative to the element, it is the orientation of a rotating magnetic field experienced by the MR element rather than the magnitude of an almost uni-directional magnetic field which causes a change in the electrical resistance of the MR element.
It is an additional object of the present invention to provide a magnetic sensor comprising an array of MR elements and an encoder wheel which carries one or more magnetic pole arrays, which use the principle of rotating fields experienced by the MR elements as the wheel rotates to produce a much greater resolution of angular position compared to previous schemes in the prior art which employ almost uni-directional fields of varying magnitude.
It is another object of this invention to provide a magnetic encoder which produces a high resolution of the angular position of an encoder wheel, in one or more incremental channels which have a pre-determined and fixed phase relationship between them, and an optional index marker channel.
Other objects of the present invention, as well as particular features, elements, and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures.